As part of an ongoing broad program of research on the comparative metabolism of normal and neoplastic cell types, we are currently directing our attention to two enzymes whose potential importance in the dynamics of phosphate utilizaton and turnover make them highly worthy of study in relation to growth rate and degree of differentiation. These are creatine kinase (2.7.3.2.) and inorganic pyrophosphatase (3.6.1.1). Creatine kinase, in contrast with prevailing opinion, is widely distributed among animal tissues, and its regulatory function on ATP may extend beyond muscle and brain. Inorganic pyrophosphate is formed in so many and varied anabolic processes, the importance of its disposition in cells, particularly those of rapid growth rate, can hardly be overestimated. The activities, properties, and isozyme composition of these enzymes are being determined in normal tissues, and in a variety of neoplasms of different cell types, growth rate, and degree of differentiation, both as manifestations of abnormalities of gene expression in neoplasia, and as modulators of Pi, PPi and nucleoside phosphate levels. Attempts will be made to relate the loss or retention of these enzymes to the metabolic and biological behavior of the tumors as they might be affected by the levels of phosphate, pyrophosphate and nucleoside phosphates.